some more remarks

diff --git a/ralf/_posts/2019-11-25-how-to-panic-in-rust.md b/ralf/_posts/2019-11-25-how-to-panic-in-rust.md
index 2c03da8e783d84fff83201eebafb5bafe8216259..f9ac9ed96308a4d9783e6c3e7fe448b0ebcbb6fe 100644 (file)
--- a/ralf/_posts/2019-11-25-how-to-panic-in-rust.md
+++ b/ralf/_posts/2019-11-25-how-to-panic-in-rust.md
@@ -31,7 +31,7 @@ I think the best place to start are the interfaces controlling the two indirecti
 * The *panic runtime* is used by libstd to control what happens after the panic information has been printed to stderr: either we abort (`-C panic=abort`) or we unwind (`-C panic=unwind`).
   (The panic runtime also provides the implementation for [`catch_unwind`](https://doc.rust-lang.org/std/panic/fn.catch_unwind.html) but we are not concerned with that here.)
 
-* The *panic handler* is used by libcore to implement (a) panics inserted by codegen (such as panics caused by arithmetic overflow or out-of-bounds array/slice indexing) and (b) the `core::panic!` macro (this is the `panic!` macro in libcore itself and in `#[no_std]` context in general).
+* The *panic handler* is used by libcore to implement (a) panics inserted by code generation (such as panics caused by arithmetic overflow or out-of-bounds array/slice indexing) and (b) the `core::panic!` macro (this is the `panic!` macro in libcore itself and in `#[no_std]` context in general).
 
 Both of these interfaces are implemented through `extern` blocks: listd/libcore, respectively, just import some function that they delegate to, and somewhere entirely else in the crate tree, that function gets implemented.
 The import only gets resolved during linking; looking locally at that code, one cannot tell where the actual implementation of the respective interface lives.
@@ -70,7 +70,7 @@ On top of the panic *runtime* interface, libstd implements the default Rust pani
 
 #### `rust_panic_with_hook`
 
-The key function that everything passes through is [`rust_panic_with_hook`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L435):
+The key function that everything passes through is [`rust_panic_with_hook`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L435-L437):
 {% highlight rust %}
 fn rust_panic_with_hook(
     payload: &mut dyn BoxMeUp,
@@ -97,11 +97,14 @@ The `'static` bound is quite well hidden there, but after a while I realized tha
 
 `rust_panic_with_hook` is a private function to `std::panicking`; the module provides three separate entry points on top of this central function:
 
-* the [default panic handler implementation](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L301), backing (as we will see) panics from `core::panic!` and built-in panics (from arithmetic overflow or array/slice indexing checks).
+* the [default panic handler implementation](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L301), backing (as we will see) panics from `core::panic!` and built-in panics (from arithmetic overflow or out-of-bounds array/slice indexing).
   This obtains as input a [`PanicInfo` ](https://doc.rust-lang.org/core/panic/struct.PanicInfo.html), and it has to turn that into arguments for `rust_panic_with_hook`.
   Curiously, even though the components of `PanicInfo` and the arguments of `rust_panic_with_hook` match up pretty well and seem like they could just be forwarded, that is *not* what happens.
   Instead, libstd entirely *ignores* the `payload` component of the `PanicInfo`, and sets up the actual payload (passed to `rust_panic_with_hook`) such that it contains [the formatted `message`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L348).
 
+    One consequence of this setup is that, as far as I can tell, the `-C panic` option has no effect at all for `no_std` applications!
+  It only comes into play when libstd's panic handler implementation is used.
+
 * [`begin_panic_fmt`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L319), backing the [format string version of `std::panic!`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/macros.rs#L22-L23) (i.e., this is used when you pass multiple arguments to the macro).
   This basically just packages the format string arguments into a `PanicInfo` (with a [dummy payload](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panic.rs#L56)) and calls the default panic handler that we just discussed.
 
@@ -131,9 +134,14 @@ No formatting happens here as that would require heap allocations; this is why `
 
 Curiously, the `payload` field of the `PanicInfo` that gets passed to the panic handler is always set to [a dummy value](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panic.rs#L56).
 This explains why the libstd panic handler ignores the payload (and instead constructs a new payload from the `message`), but that makes me wonder why that field is part of the panic handler API in the first place.
-Another consequence of this is that `core::panic!("message")` and `std::panic!("message")` (the variants without any formatting) actually result in very different panics: the former gets turned into `fmt::Arguments`, passed through the panic handler interface, and then libstd creates a `String` payload by formatting it.
+Another consequence of this is that [`core::panic!("message")`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L15) and [`std::panic!("message")`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/macros.rs#L16) (the variants without any formatting) actually result in very different panics: the former gets turned into `fmt::Arguments`, passed through the panic handler interface, and then libstd creates a `String` payload by formatting it.
 The latter, however, directly uses the `&str` as a payload, and the `message` field remains `None` (as already mentioned).
 
+Some elements of the libcore panic API are lang items because the compiler inserts calls to these functions during code generation:
+* The [`panic` lang item](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs#L39) is called when the compiler needs to raise a panic that does not require any formatting (such as arithmetic overflow); this is the same function that also backs single-argument [`core::panic!`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L15).
+* The [`panic_bounds_check`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs#L55) lang item is called on a failed array/slice bounds check.
+  It calls into the same method as [`core::panic!` with formatting](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L21-L24).
+
 ## Conclusion
 
 We have walked through 4 layers of APIs, 2 of which are indirected through imported function calls and resolved by the linker.